Blade mount and stall control for vane axial compressors

ABSTRACT

Radial compressor blades are adjustably and releasably mounted on the impeller hub. A set of rigid bayonet formations defines the blade axial position positively at all pitch angles. A set of flexible detent formations defines the pitch angle effectively positively during operation, but yieldably for manual adjustment between discrete values. A positive lock must be released for blade removal. Especially at low pitch angles, the compressor is stabilized against stall by aperturing one or more blades.

United States Patent Inventors Robert E. Salkbury Whittier; John W.Erickson, Huntington Beach, both of, Calif. App]. No. 842,983 Filed July18, 1969 Patented Aug. 3, 1971 Assignee Preco, Inc.

Los Angeles, Calil.

BLADE MOUNT AND STALL CONTROL FOR VANE AXIAL COMPRFSSORS 6 Claims, 10Drawing Figs.

US. Cl 416/206, 416/221, 416/231 Int. Cl F0ld 5/30 Field of Search416/206,-

[56] References Cited UNITED STATES PATENTS 1,066,988 7/1913 Boutwell416/231 1,888,056 11/1932 Verzillo et al. 416/231 X 3,026,943 3/1962Huber 416/206 3,428,244 2/1969 Palmer 416/231 X PrimaryExaminer-Everette A. Powell, Jr. Attorney-Charlton M. Lewis ABSTRACT:Radial compressor blades are adjustably-and releasably mounted on theimpeller hub. A set of rigid bayonet formations defines the blade axialposition positively at all pitch angles. A set of flexible detentformations defines the pitch angle effectively positively duringoperation, but yieldably for manual adjustment between discrete values.A positive lock must be released for blade removal.

Especially at low pitch angles, the compressor is stabilized againststall by aperturing one or more blades.

BLADE MOUNT AND STALL CONTROL FOR VANE AXIAL COMFRESSORS This inventionrelates generally to axial flow fans, blowers and gas compressors, whichwill be referred to collectively as compressors. The invention providesimproved structure for mounting the impeller blades on the fan hub, andalso provides improved stability with respect to aerodynamic stalling.

One important object of the invention is to provide structure by whichthe pitch angle of the mounted impeller blades can be adjustedconveniently, while insuring reliable locking of the blades in adjustedposition. The invention further permits convenient and rapid disassemblyof one or more blades individually, as for inspection or replacement.

A further object of the invention is to provide mounting structure forimpeller blades that is more economical to manufacture than previouslyknown structures, while insuring accurate and reproducible positioningof each blade with respect to the fan hub.

The invention has the particular advantage that each blade is shiftableonly among a finite number of discrete pitch angles, each of which ispositively and precisely defined by the mechanical mounting structure,with clear indication by reference marks of improved type. All of theblades can therefore be shifted from one position to another withoutrequiring any precision tools, yet with full confidence that theadjusted angles of all blades are accurately equal.

Another aspect of the invention improves fan operation throughout a widerange of blade adjustment by reducing the tendency to abrupt aerodynamicstalling. It is well known that axial compressors are subject toinstability as the airflow is throttled, typically shifting abruptly toa stalled condition with a significant drop in output pressure. Thattendency varies in severity as the blade pitch angle is changed,ordinarily becoming more serious as the openings between blades areclosed down.

The present invention increases the range of stable operation byproviding one or more paths for backward gas flow between the pressureside and the suction side of individual blades. Such paths are typicallyprovided via apertures in the blades, a single aperture near the bladetip being normally sufficient. One or more blades may be so apertured.When a plurality of blades are apertured, they are preferably selectedsymmetrically with respect to the blower axis. Provision of a smallnumber ofsuch paths for backflow, typically from one to six, reduces thetotal pressure produced by the fan only slightly, and typically has nomeasurable effect on the normal flow. The resulting increase in therange of stable operation, although advantageous in any compressor, isespecially valuable in combination with an impeller having adjustableblades.

The prior art includes many suggested structures for adjustably mountingimpeller blades on a hub. Many of those structures require accurate andeven elaborate machining of parts, and are therefore expensive toconstruct. Others require manipulation of locknuts within the impellerhub, or of setscrews, before the blades can be adjusted, followed bytightening of those elements after adjustment. With such structures,quite aside from the labor and time required to set and lock the blades,it is extremely difficult to position all blades at accurately identicalpitch angles. Still other proposed structures provide convenientadjustment of blade pitch between only two or three alternative values,or hold the blades so loosely that they tend to vibrate duringoperation. Other previously suggested structures permit individualadjustment of the blades, but not their removal.

Those and other disadvantages of the prior art are overcome by structurethat includes mutually complementary formations of bayonet type mountedessentially rigidly on the blade and on the fan hub, the axis of themount structure being typically perpendicular to the compressor axis andessentially coinciding with the longitudinal axis of the blade. Thatrigid bayonet structure positively restrains the assembled blade againstradially outward movement from the hub, rigidly defining the directionof the longitudinal blade axis relative to the hub, while leaving theblade essentially free to rotate within a limited range of rotaryadjustment about the mount axis. When the blade is rotated to the limitof that range of adjustment in one direction the rigid bayonet structurereleases the blade and it may be freely removed from the hub in a radialdirection.

The invention further provides locking means for retaining the blade inany selected one of a plurality of distinct, angularly spaced, rotarypositions. Such locking means preferably comprise resilient detentstructure for positively and accurately defining each discrete position,while permitting the blade to be shifted from one position to anothermerely by manual application of a suitable torque.

In preferred form, the invention further provides structure for lockingthe blade essentially positively to prevent its rotation from theangular range of its selectively adjustable positions to the releaseposition of the bayonet formations. That locking structure typicallycomprises a latch member that engages a positive stop to prevent suchrelease of the blade, but is resiliently deflectable out of the path ofthat stop, as by application of a suitable tool, when it is desired toremove a blade from the fan hub.

A full understanding of the invention, and of its further objects andadvantages, will be had from the following description of certainillustrative manners of carrying it out, which description is to be readwith relation to the accompanying drawings, in which FIG. 1 is atransverse section of an illustrative compressor impeller embodying theinvention, as seen from the pressure side;

FIG. 2 is a schematic graph illustrating the effect of one aspect of theinvention on the stall characteristic of a typical compressor;

FIG. 3 is a fragmentary section of line 3-3 of FIG. 1 at enlarged scale,representing one embodiment of another aspect of the invention;

FIG. 4 is a section on line 4-4 ofFIG. 3;

FIG. 5 is an elevation of the hub with blade removed, in the same aspectas FIG. 3;

FIG. 6 is a section on line 6-6 at further enlarged scale;

FIG. 7 is a fragmentary elevation in the aspect indicated by line 7-7ofFlG. 1;

FIG. 8 is a schematic perspective, showing a mounted blade and anunmounted blade;

FIG. 9 is a section corresponding to FIG. 3 but looking radially outwardfrom inside the hub and representing a modification; and

FIG. 10 is a section on line 10-10 of FIG. 9.

In FIG. 1 an axial flow compressor is shown in transverse section, asseen from the pressure side, with impeller 20 mounted on the shaft 22and enclosed in the cylindrical housing 24. Shaft 22 is joumaled on theaxis 21 and is driven in conventional manner, not explicitly shown.Impeller 20 comprises the hub 26 on which the blades 30 are mounted bymechanism to be more particularly described. The removable plate 36provides access to the blades one at a time through the aperture 34 inhousing 24.

In accordance with one aspect of the invention, one or more selectedblades are apertured, the apertures being most effective when near theblade tip and closer to the leading edge than to the trailing edge ofthe blade, as illustratively shown at 32, 32a and 32b. When thecompressor is operated under design conditions, the apertures or ports32 are found to have only negligible effect on the total pressure thatis produced, typically lowering that pressure by l or 2 percent. Thereis ordinarily no measurable effect upon the total flow. However, as theflow is reduced by throttling of the output, the abrupt aerodynamicstalling that is characteristic of axial compressors under suchconditions is greatly reduced or entirely eliminated.

In FIG. 2 the dashed lines are a schematic plot of total pressure riseas a function of flow for constant speed operation of a typicalconventional single-stage compressor. As the weight flow is reduced bythrottling from the normal operating region A to the critical region Bbelow the peak pressure, the total pressure tends to decrease abruptly,as at C, to a lowered value, shown by the curve D. If the flow is thenagain increased, the compressor recovers abruptly, but on the line E,which corresponds to a higher flow rate than that at which the decreaseC occurred. Hence there is a hysteresis region of unstable operation,represented in the present illustrative diagram by the area enclosed bylines B, C, D and B. As is well known, such instability can lead toserious difficulties and even dangers, depending upon the function ofthe particular compressor.

In contrast, an impeller provided with antistall ports in accordancewith the present invention typically produces the stable operationrepresented by the solid line F in FIG. 2. The total pressure riseproduced in the normal operating region A is only slightly reduced. Thatreduction increases gradually as the flow is throttled, but typicallymerges smoothly and without any sharp break into curve D, the normaloperating characteristic for low flow. As the flow is increased, thepressure recovers along the same line F, without hysteresis.

The phenomenon of abrupt stall is not well understood, and no successfulanalytical treatment has been proposed. Therefore no detailed analysiscan be given of the described action of the antistall ports 32 of thepresent invention. However, that action is believed to be due, at leastin part, to the production of a local stall condition adjacent each portearly in the throttling phase. Such local stall regions may then spreadgradually to affect the entire fan, eliminating the instability andaccompanying hysteresis that are normally encountered.

A remarkable feature of the described antistall apertures is that onlyone such aperture is sufficient to extend the region of stable operationvery significantly, and may produce complete stability as shown in FIG.2. When several apertures are used, they have been found most effectivewhen located in blades that are angularly spaced in a symmetricalpattern. Such arrangement probably tends to stimulate the normal stallpatterns in the blading, which usually occur symmetrically with respectto the axis. The number of blades per row is usually selected as a primenumber, which is necessarily odd. FIG. 1 illustrates an impeller with 13blades of which three are apertured, forming two blade groups that arearranged symmetrically. Although apertures 32 are shown typically asholes within the blade boundaries, the term aperture" is intended toinclude openings that intersect the blade edge.

A preferred structure for adjustably mounting each of the impellerblades 30 is shown somewhat schematically in FIGS. 3 to 8. A section ofthe cylindrical hub shell 28 is flattened at each blade, and the flatarea 29 is punched to form an aperture 40 of a pattern typically asshown in FIG. 5. Aperture 40 is generally circular with primary radiumr, and has three angularly spaced bays 41, 42 and 43 and six slots 44 to49, which extend radially outward from the circular aperture edges. Bay41 is somewhat narrower and radially deeper than bays 42 and 43. Two ofthe slots 44 to 49 are positioned between each pair of adjacent bays 41to 43. As will become apparent, those slots may be replaced, if desired,by channels or recesses of any desired form, and may be provided in anydesired number.

Blade 30 carries at its base end 33 the rigid disk like bayonet element50, and the slightly flexible detent element 60 of resilient sheetmaterial. Elements 50 and 60 are typi ally welded to the blade end, asindicated at 56, in parallel adjacent relation, perpendicular to thelongitudinal blade axis, indicated at 39. Element 60 may be slotted at61 to insure firm bonding of both elements. The shape of bayonet element50 is Each of the bayonet wings 51 to 53 is offset as shown best in.FIG. 3, just sufficiently to engage the inner face of hub shell 28 whenrotated after full insertion through aperture 40. The wings preferablylie flatly in a common plane and do not bias rotation of the bayonetmember preferentially in either direction. The wing offset is relativelysharp, and is dimensioned to provide a free fit within radius r of hubaperture 40, as indicated at 55 in FIG. 4, accurately guiding bladerotation. The magnitude of the offset is so related to the thickness ofthe hub sleeve that the latter fits closely between the offset wings andthe butt end 33 of the blade where it overlies the hub, positivelymaintaining coaxial relation of the bayonet fittings on the common axis38, but permitting free blade rotation. As a further indexing device, apositive stop is provided to prevent the inserted bayonet element 60from being rotated clockwise as seen in FIG. 3. Such a stop is shown at57, comprising an inwardly bent ear on the edge of bay 41.

Detent element 60 is generally circular and directly overlies the outerface of hub sleeve 28. Element 60 carries three angularly spaced detentformations 62, 63 and 64, which comprise radial ridges on the inner faceof the element. Those ridges are of such dimensions as to enter theradial channels 44 to 49 of aperture 40, engaging the edges of thosechannels obliquely and camming the entire assembly to a definite rotaryposition (FIG. 6). Each of the detent formations 62 to 64 cooperateswith one pair of the radial channels 44 to 49, and is thus capable ofdefining two distinct rotary positions of the assembly. However,channels 44 to 49 are angularly staggered in such a way that only onedetent formation engages a channel in any one assembly position. Thusthe detent mechanism is capable of defining six closely adjacent rotarypositions, even though each of the detent formations occupies arelatively large sector and can therefore be made rugged in structureand both positive and accurate in its action. Those detent formations 62to 64 which are not engaged in a slot are supported in slightlydeflected position on the outer hub surface. Thus all three detentformations exert resilient force on the hub, urging the blade outwardand maintaining firm contact of bayonet wings 51 to 53 against the innerhub face.

In operation of the blower, centrifugal force urges each blade stronglyaway from axis of rotation 21. That force is taken by bayonet element50, being transferred through its rigid and sturdy wings 51 to 53 to theinner hub face. In practice it has been found that at normal operatingspeed the blade is maintained with its longitudinal axis in properposition relative to the hub entirely by that action of bayonet element50 and does not depend upon the resilient action of detent element 60.The resilient stiffness of the latter is therefore required primarily toprovide reliable definition of the pitch angle of the blade. For a bladethat is properly balanced with respect to mounting axis 38, suchdefinition requires relatively little torque.

As a convenience during blade adjustment, and as assurance against lossof a blade in case the described detent system should fail for anyreason, it is desirable to provide a protective latch mechanism forpreventing rotation of bayonet element 50 to blade-releasing position.In the present embodiment such a latch comprises the radial arm which isformed on detent element 60 in the space between the adjacent two detentformations 63 and 64. Arm 70 is bent toward the hub and enters bay 42 ofhub aperture 40, as shown best in FIG. 4. During assembly of a blade,arm 70 strikes the outer hub face in the vicinity of detent slots 44 and47 and can slide freely over that face and over the detent slots untilit reaches bay 42. The arm then resiliently springs into that bay,positively limiting rotation of the assembly to the angular rangedefined by the dimensions of the bay and arm. In order to disassemblethe blade, a suitable tool is inserted through access aperture 34 andhooked under arm 70, and the arm is lifted sufficiently to swing overthe outer face of the hub. The assembly may then be rotated freely toreleasing position.

The access aperture 34 in housing 24 is covered by the removable coverplate 36, providing convenient access for assembly, adjustment, orremoval of blades 30, the shaft being rotated to bring the bladessuccessively into alignment with the aperture. Scale marks arepreferably provided on housing 24 at the periphery of aperture 34 forreading directly the pitch adjustment of each blade in turn. Correctcentering of 5 the blade in aperture 34 is insured by providing a scalefor each blade edge, as shown in FIG. 7 at 37 and 37a, and setting theimpeller angle so that both scales read the same. Scale indications,designated R in FIG. 7, preferably show the release position of theblade.

FIGS. 9 and represent a modified structure in accordance with theinvention, in which generally corresponding parts are designated withthe same numerals with addition of 'a letter a. FIG. 9 correspondsgenerally to FIG. 3 of the firstdescribed embodiment, but in oppositeaspect, showing the structure as seen from within the hub. In FIGS. 9and 10 the resilient detent element 600 is mounted on the opposite faceof rigid bayonet element 50a from blade 30a. The ears 80 are punched outof element 59a and are welded to the blade, while element 600 may bespotwelded to element 500 as indicated at 81.

Bayonet element 500 carries the wings 51a, 52a and 53a which engage theinner face of hub shell 28a as shown in the drawings, and which can passthrough the respective bays 41a, 42a and 43a of hub aperture 40a whenthe blade is rotated to bayonet-releasing position.

Detent element 60a carries three resilient arms with the cam formations61a, 62a and 630 at their ends. Those cam formations cooperate with theradial detent channels 44a through 480, which are formed in the presentembodiment in the periphery of the aperture bays, rather than in theinner periphery of the mounting aperture, as in the previous embodiment.Only five detent channels are shown, providing five different angles ofblade pitch, but any desired number may be provided. During insertionand removal of the blade, the wings of detent element 60a pass throughthe special bays 82, 83 and 84 of mounting aperture 40a.

Air leakage through the portions of mounting aperture 40 that are notcovered by the bayonet formations is preferably prevented by providing acover plate 90, which is typically circular and may be mounted betweenthe blade end and bayonet element 50a. Apertures 92 in the cover plateaccommodate the mounting ears 80 and locate the plate; A gasket may beprovided near the periphery of the cover plate ifdesired, but is notordinarily required. A similar cover plate may be used with thepreviously described embodiment.

In the present embodiment the mounted blade is locked against rotationto blade-releasing position by the corrugation 700 formed in one of thearms of detent element 60:: in position to project into the bay 43a ofthe mounting aperture, as shown best in FIG. 10. To release the blade,that detent arm is deflected toward the impeller axis, as by inserting atool through housing aperture 34, through the small hole 94 in coverplate 90 and through bay 43a.

A particular advantage of the illustrated structures is that themounting apertures in the hub shell, though of special shape, can beformed by a simple punching operation. The resilient detent element,being carried by the blade, is readily replaceable with the blade. Onthe other hand, it will be evident that many variations may be made inthe illustrative structures that have been described without departingfrom the proper scope of the invention. For example, the detent channels44a to 48a of FIGS. 9 and 10 can, by simple transposition of parts, beformed in the periphery of bayonet element 50a, and can then be engagedby cam formations carried by flexible detent arms mounted on the hubshell at each mounting aperture and projecting radially inward withrespect to that aperture. As a further example, although the describedcamwise engagement of the resilient detent formations for defining theblade pitch angle has the advantage of facilitating blade adjustment,those formations can readily be modified to provide positive engagementwith the fixed channels or their equivalent. Such positive engagementcan be released to permit blade adjustment by insertion of a suitabletool through housing access aperture 34, in the manner already describedfor releasing locking formation 70.

We claim:

1. In an axial flow compressor having a hub and a plurality of bladesthat are releasably mountable on the hub with each blade extendinggenerally radially from the hub, structure for mounting each blade onthe hub and comprising in combination complementary bayonet formationsmounted on the blade and on the hub on respective mounting axes that arelongitudinal of the blade and generally radial with respect to the hub,said formations being engageable in coaxial relation of the mountingaxes to define an assembled position of the blade relative to the hubalong the length of the mounting axis and to positively prevent blademovement outward from the hub along the mounting axis away from thatassembled position while permitting essentially free blade rotation withrespect to the mounting axis throughout a limited angular range, saidformations being releasable in response to blade rotation beyond saidrange in one direction to'permit removal of the blade,

and retaining means acting between the blade and the hub for defining aplurality of mutually spaced angular operating positions of the bladewithin said limited range and for releasably retaining the blade in aselected one of said operating positions.

2. Blade-mounting structure as defined in claim 1, and in which i saidretaining means are resiliently releasable inresponse to application tothe blade of rotational torque with respect to said mounting axis, suchtorque exceeding the maximum torque acting on the blade during normaloperation of the fan.

3. Blade-mounting structure as defined in claim I, and in which saidretaining means comprise a plurality of first detent formations mountedon one of said blade and hub and forming at least two groups offormations that are angularly spaced by more than said angular range,the formations of each group being angularly. spaced by less than saidangular range and by more than the spacing of adjacent operatingpositions of the blade,

and a plurality of second detent formations mounted on the other of saidblade and hub and angularly spaced in general correspondence torespective groups of said first detent formations,

the first and second detent formations being adapted for selectivemutual engagement to define said plurality of distinct angular operatingpositions of the blade, the mutual angular spacing of said formationsbeing such that adjacent operatingpositions of the blade correspond toengagement of difierent second detent formations with first detentformations of their respective groups.

4. Blade-mounting structure as defined in claim 1, and including alsolocking structure normally acting to positively prevent blade rotationfrom said limited angular range to said blade-releasing position,

said locking structure being resiliently defiectable to release theblade for such rotation.

5. Blade-mounting structure as defined in claim 4, said retaining meansand said locking structure comprising a common element mounted on one ofthe blade and hub and including first and second resiliently deflectableportions,

a detent formation mounted on the other of the blade and hub andengageable camwise by said first portion to retain the blade yieldablyin an operating position,

and a stop formation mounted on said other of the blade and hub andnormally engageable by said second portion to positively prevent bladerotation from said limited angular rangeto said blade-releasingposition, said second portion being manually defiectable to release saidstop formation.

at least one blade is apertured to form a path for gas flow between thepressure side and the suction side of the blade.

1. In an axial flow compressor having a hub and a plurality of bladesthat are releasably mountable on the hub with each blade extendinggenerally radially from the hub, structure for mounting each blade onthe hub and comprising in combination complementary bayonet formationsmounted on the blade and on the hub on respective mounting axes that arelongitudinal of the blade and generally radial with respect to the hub,said formations being engageable in coaxial relation of the mountingaxes to define an assembled position of the blade relative to the hubalong the length of the mounting axis and to positively prevent blademovement outward from the hub along the mounting axis away from thatassembled position while permitting essentially free blade rotation withrespect to the mounting axis throughout a limited angular range, saidformations being releasable in response to blade rotation beyond saidrange in one direction to permit removal of the blade, and retainingmeans acting between the blade and the hub for defining a plurality ofmutually spaced angular operating positions of the blade within saidlimited range and for releasably retaining the blade in a selected oneof said operating positions.
 2. Blade-mounting structure as defined inclaim 1, and in which said retaining means are resiliently releasable inresponse tO application to the blade of rotational torque with respectto said mounting axis, such torque exceeding the maximum torque actingon the blade during normal operation of the fan.
 3. Blade-mountingstructure as defined in claim 1, and in which said retaining meanscomprise a plurality of first detent formations mounted on one of saidblade and hub and forming at least two groups of formations that areangularly spaced by more than said angular range, the formations of eachgroup being angularly spaced by less than said angular range and by morethan the spacing of adjacent operating positions of the blade, and aplurality of second detent formations mounted on the other of said bladeand hub and angularly spaced in general correspondence to respectivegroups of said first detent formations, the first and second detentformations being adapted for selective mutual engagement to define saidplurality of distinct angular operating positions of the blade, themutual angular spacing of said formations being such that adjacentoperating positions of the blade correspond to engagement of differentsecond detent formations with first detent formations of theirrespective groups.
 4. Blade-mounting structure as defined in claim 1,and including also locking structure normally acting to positivelyprevent blade rotation from said limited angular range to saidblade-releasing position, said locking structure being resilientlydeflectable to release the blade for such rotation.
 5. Blade-mountingstructure as defined in claim 4, said retaining means and said lockingstructure comprising a common element mounted on one of the blade andhub and including first and second resiliently deflectable portions, adetent formation mounted on the other of the blade and hub andengageable camwise by said first portion to retain the blade yieldablyin an operating position, and a stop formation mounted on said other ofthe blade and hub and normally engageable by said second portion topositively prevent blade rotation from said limited angular range tosaid blade-releasing position, said second portion being manuallydeflectable to release said stop formation.
 6. A rotary, axial flow gascompressor having its blades mounted by the blade-mounting structuredefined in claim 1 for releasably retaining the blades in a selectedoperating position, and in which at least one blade is apertured to forma path for gas flow between the pressure side and the suction side ofthe blade.